Forging steel for elevated temperature service



United States Patent 3,316,084 FORGING STEEL FOR ELEVATED TEMPERATURESERVICE Samuel J. Manganello, Penn Hills Township, Allegheny County,Pa., assignor to United States Steel Corporation, a corporation ofDelaware No Drawing. Filed May 18, 1964, Ser. No. 368,404 1 Claim. (Cl.75-128) This invention relates to improvements in forging steels forelevated temperature service and more particularly to steels suitablefor heavy forgings such as steam-turbine rotors.

Steam rotor forgings are heavy forgings ranging up to several feet indiameter and which must withstand temperatures of the order of 1100 F.in service. Also these forgings must meet exacting specificationsincluding a minimum room temperature yield strength of about 85,000 psi,a minimum elongation of about 14 percent, a minimum reduction of area ofabout 35 percent, a maximum Charpy V-notch 50-percent shear fracturetransition temperature of 250 F., and good stress-rupture life in therange of 950 to 1200 F. combined with notch insensitivity duringstress-rupture testing. In addition, steam-turbine rotors must bedimensionally stable at the operating temperatures of the intended use.

It is accordingly an object of this invention to provide a steel forheavy forgings for elevated temperature use having the above mentionedproperties.

It is a further object of this invention to provide steamturbine rotorshaving the above mentioned properties.

I have discovered that a steel containing carbon, nickel, chromium,vanadium, molybdenum and manganese with- Chromium 0.85/ 1.45 Vanadium020/065 Manganese -0.30/1.10 Molybdenum 0.90/225 Nitrogen max 0.015Silicon max .40 Aluminum (acid soluble) max 0.05

balance iron and residual impurities. Within such compositional rangesthe manganese and molybdenum should be balanced so that with less than0.75 percent manganese the molybdenum should be at least 1.6 percent andwith .75 percent or more manganese, the molybdenum should not exceed 1.6percent. As indicated the steel may contain deoxidizers such as siliconand aluminum in amounts up to 0.40 and 0.05 percent respectively.

Thus within the foregoing ranges of my invention, there are twopreferred analyses in weight percent:

Carbon 0. /0. 0. 25/0. 3 Nickel. 0. 70/1. 25 0. 70/1. 25 Chromium 0.85/1. 0. 85/1. 45

Vauadiuin. 0. 20/0. 0. 20/0. 65 Manganese. 0. 30/0. 0. 75/1. 10Molybdenum 1. 6/2. 25 0. /1. 6 Nitrogen, maximuin 0. 015 0. 015 Silicon,maximum .40 40 Aluminum, maximum 05 05 with the balance iron andresidual impurities.

The benefits of this invention are illustrated by steels of thecompositions in weight percent listed in the following Table I:

TABLE I Steel C Mn P S Si I Ni Cr Mo V Al* N 0. 28 0.31 0. 012 0. 011 0.28 l. 03 1. 18 2. 04 0.25 0. 005 0. 006 0. 28 0. 30 0. 014 0. 011 0. 28l. 07 1.18 1. 67 0. 47 O. 006 (J. 008 0. 29 0. 87 0.012 0. 014 0. 29 0.78 1.15 1. 09 0. G4 0. 006 0. 008 0. 29 0. 30 0. 012 0. 011 0. 27 0. 761.15 1. 97 t]. 28 0. 004 0. 008

*Acid soluble.

in the hereinafter set forth restricted ranges and wherein the manganeseand molybdenum are critically proportioned will provide theaforementioned required properties. The composition of my invention hasthe desirable qualities of not forming pearlite or proeutectoid ferriteafter normalizing, good notch toughness, room temperature yield strengthand elevated temperature stress-rupture properties. The compositionalranges (percent by weight) of my improved steel are:

Forged longitudinal specimen blanks of the foregoing steels wereaustenitized for four hours at 1775 C., cooled to 600 F. at about F. perhour (approximately the cooling rate at a location 3 inches from thesurface of an air-cooled 45-inch diameter rotor forging), furnace cooledto room temperature, tempered for 24 hours at the temperature indicatedin the following Table II and thereafter furnace cooled to roomtemperature. The room temperature tension test values and the CharpyCarbon 0.25 /-0.35 V-notch impact values for these steels so treated aregiven Nickel 0.70/ 1.25 in Table II:

TABLE II Charpy Yield Reduction Energy V-notch Steel Tampering Strength,Elong. in 2, of area, Absorbed 50% Shear Temp, F. p.s.i., 0.2% PercentPercent at 75 F., Fracture- Ottset tt.-1b. Transition Temp, F.

The stress-rupture test results (obtained with combinationsmooth-notched creep rupture specimens) on two of the steels are listedin the following Table III:

itizing, tempered, and furnace cooled to room temperature.

A review of the data indicates that the steel composition of thisinvention produces the desired properties for TABLE III 1,000 F., 60,000p.s.i. 1,000 F., 50,000 p.s.i. 1,100 F., 40,000 p.s.i. Steel TIRJ El.RA. T.R. El. R.A. T.R. El. RA.

1,000 F., 30,000 p.s.i. 1,200 F., 20,000 p.s.i. 1.250 F., 15,000 p.s.i.Steel T.R. El. R.A. T.R. El. RA. I.R. El. RA.

1 Time torupture in hours. 1 Elongation 111 1 inch. 3 Reduction of area.

*Speeimen blanks were austenitized for 4 hours at 1,850 F. and furnacecooled to room temperature before being subjected to the above heattreatment.

The normalized steels had a uniform microstructure consisting of anintimate mixture of ferrite plus bainite. The steel may be single ordouble-normalized (homogenized then normalized) and still exhibit thegood combination of microstructure and properties. Thus the steel hassufficient hardenability to permit heavy forgings to cool duringnormalizing without forming proeutectoid ferrite and pearlite which havean undesirable effect on strength and impact properties.

I have found that even with the specified nickel content, manganese andmolybdenum must be critically balanced to achieve the desired improvedmechanical properties. In evaluating the various test results, it wasobserved that when the manganese content dropped below a certain level,the steel was not satisfactory unless the molybdenum content was above acertain level. Further investigation revealed that the converse was alsotrue. Thus, when the manganese is in the range 0.30 to 0.75 percent themolybdenum should be 1.6 to 2.25 percent, and when the manganese is inthe range 0.75 to 1.10 percent the molybdenum should be less than 1.6percent. The silicon content should not be much higher than 0.40percent, because high amounts of silicon are deleterious tostress-rupture properties. Higher amounts of nickel and vanadium arebeneficial to notch toughness (impact properties). In fact, increasingthe nickel content appears to be also beneficial to the tensile andstressrupture properties of this steel. Higher amounts of molybdenum(and to a certain extent, vanadium) are beneficial to the stress-ruptureproperties. The carbon content must be kept within a rather narrow range(0.26 to 0.35 percent) to maintain high yield strength and reasonablygood notch toughness.

The heat treatment to which these chromium-molybdenum-vanadium steelsare subjected is critical. The use of final austenitizing temperatureshigher than about 1800 F. is detrimental to notch toughness; theimpairment to notch toughness is caused chiefly by austenite graincoareening. However, the time at austenitizing temperature will varywithin the conventional range of to hours depending upon thecross-sectional area of the forging under treatment. The temperingtemperature is maintained within the range of 1225 to 1280 F. for timesof 20 to 40 hours at temperature. As is conventional, the Steel is aircooled (in still or agitated air) after austensteam-turbine rotors andother heavy forgings for elevated temperature service. Thus the steelhas a room temperature 0.2 percent offset yield strength in excess of85,000 p.s.i., a fracture transition temperature less than 250 F., goodresistance to temper embrittlement, very good stress-rupture propertiesat temperatures up to 1200 F., and is notch insensitive. As previouslystated, the steel has a microstructure consisting of a mixture offerrite and bainite.

While I have shown and described several specific embodiments of myinvention, it will be understood that these embodiments are merely forthe purpose of illustration and description and that various other formsmay be devised within the scope of my invention, as defined in theappended claim.

I claim:

A heavy forging steel for elevated temperature service characterized bya minimum room temperature yield strength of 85,000 p.s.i., a maximumCharpy V-notch 50 percent shear fracture transition temperature of 250F. and good stress-rupture properties in the range of 950 to 1200 B,said steel consisting essentially of Carbon 025/035 with the balanceiron and residual impurities.

References Cited by the Examiner UNITED STATES PATENTS 1,660,790 2/1928Herman l28 X 2,798,805 7/1957 Hodge 75--12 8 X 2,921,849 1/ 1960Furgason 75-128 HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

P. WEINSTEIN, Assistant Examiner.

